Improved the performance and intelligence of resource harvesting by

refactoring the Harvesters' pathfinding. Now they in first place assess
which is the closest resource inside their search area and then a path is
calculated

Changed the way harvesters find resources by always trying to find the
closest resource to their refinery.

Changed the strategy of finding to find resources in Annulus.

OpenRA.Mods.Common/Pathfinder/Constants.cs

@@ -25,5 +25,7 @@ namespace OpenRA.Mods.Common.Pathfinder
 		/// a unit took to move one cell diagonally)
 		/// </summary>
 		public const int DiagonalCellCost = 177;
+
+		public const int InvalidNode = int.MaxValue;
 	}
 }